Permselective membranes for gas separation are known, and commercial gas-separation membranes are beginning to challenge conventional technology in such areas as the production of oxygen-enriched air, nitrogen production for blanketing and other industrial applications, separation of carbon dioxide from methane, and hydrogen recovery from various gas mixtures.
The principal current types of high-performance gas-separation membranes have developed from the anisotropic, cellulose acetate, reverse-osmosis membranes of Loeb and Sourirajan. (S. Loeb and S. Sourirajan, "Sea Water Demineralization by Means of an Osmotic Membrane", ACS Advances in Chemistry Series 38, 117 (1963)) It is possible to make membranes with good characteristics in this way, and gas-separation membranes of this type have found some commercial application. However, a limited number of polymers that can be used to make anisotropic membranes with useful gas fluxes and selectivities is known.
An alternative approach is to coat a Loeb-Sourirajan anisotropic membrane with a thin, permeable sealing layer as disclosed in U.S. Pat. No. 4,230,463 to Henis and Tripodi. The sealing layer, typically silicone rubber, does not function as a selective barrier, but rather serves to plug defects in the permselective membrane, and reduce to negligible amounts the gas flow through these defects. Because the selective layer no longer has to be completely defect-free, membranes of this type can be made thin more easily than regular Loeb-Sourirajan membranes. The increase in flux that is possible with a very thin permselective layer more than compensates for the slight loss in flux caused by the presence of the sealing layer. The selective layer must still be sufficiently thick to withstand normal operating pressures in use.
A third type of gas separation membrane is a composite structure comprising a high permeability porous support membrane and a thin permselective membrane. In contrast to Loeb-Sourirajan membranes, the strength function is separated from the permselective function in this case. U.S. Pat. No. 4,243,701 to Riley and Grabowsky describes such membranes, as does a paper by Ward et al. (W. J. Ward III, W. R. Browall and R. M. Salemme, "Ultrathin Silicone/Polycarbonate Membranes for Gas Separation Processes", J. Memb. Sci. 1, 99 (1976). A disadvantage encountered with these membranes is that a defect-free coating cannot be obtained without making the permselective layer relatively thick, and consequently relatively low-flux.
Another possible membrane structure is a three-layer composite in which strength, sealing and permselective functions are all separated and performed by different elements of the composite. The membrane substrate layer is a finely microporous support film that has no permselective properties but gives mechanical strength to the composite system. This substrate is coated with a thin rubbery sealing layer which plugs the support defects and provides a smooth surface onto which the top layer may be coated. With this configuration, the permselective layer may be extremely thin, and the resulting composite membrane can produce high permeate fluxes at modest pressures. U.S. Pat. No. 3,874,986 to Browall and Salemme discloses a membrane of this type with a permselective layer of polyphenylene oxide. Japanese Laid-Open Application No. 59-59214 describes another such membrane in which the permselective layer is polymethylpentene, and Japanese Laid-Open Application No. 59-112802 gives an example of this type of composite with polybutadiene permselective coating. It is also known in the art to apply yet another permeable coating on top of the permselective layer to protect it from physical damage, and composites of this type are disclosed for example in Japanese Laid-Open Applications No. 59-66308 and 60-137418. The general concept of coating a composite membrane with a rubbery top layer is disclosed in U.S. Pat. No. 3,980,456 to Browall. Multilayer composite membranes as described above give good results, but are more complex and costly to manufacture than simpler structures. It may be possible to design membrane configurations that give excellent results in small-scale test stamps, but it is very difficult to produce these membranes in large sheets or rolls suitable for commercial use.
The teachings of the art also include diverse methods for making permselective membranes and membrane elements. Asymmetric Loeb-Sourirajan membranes are normally made by a phase-inversion casting process. Sealing or selective layers may be coated on a microporous support by solvent evaporation. U.S. Pat. No. 4,243,701 to Riley and Grabowski, for example, teaches a method of casting a thin permselective film on the surface of a porous support membrane by a solvent casting technique using halogenated hydrocarbon solvents. Alternatively, films as thin as 50 Angstroms may be prepared by spreading and stretching a polymer solution on water. References describing this liquid casting method include U.S. Pat. No. 3,767,737 to Lundstrom and U.S. Pat. No. 4,132,824 to Kimura et al. The films thus made may be picked up on or laminated to a microporous support by vacuum pick-up or other techniques known in the art.
Alternatively, the permselective membrane may be polymerized directly on the support membrane by a variety of techniques. In some cases this involves coating the support with a prepolymer solution. For example, silicone rubber layers are normally applied to composites in prepolymer form and then cured to form the finished polymer. In other cases, the agents involved are not prepolymers for the finished material, so that a more basic chemical change is involved. For example, reverse osmosis membranes can be made by interfacial polymerization, as disclosed for instance in U.S. Pat. Nos. 4,277,344 to Cadotte or 4,559,139 to Uemura and Kurihara. U.S. Pat. No. 4,581,043 to van der Scheer covers a method of making gas separation membranes where the ultrathin selective layer is formed by plasma polymerization directly on the support. U.S. Pat. No. 4,440,643, to Makino et al. describes a method for making polyimide membranes by coating a polyimide support with a solution of polyamic acid, then treating the coated substrate at high temperature to convert the polyamic acid by imide-cyclization to a polyimide.
In spite of the considerable research effort in separation membranes in recent years, few sucessful commercial gas separation membranes have been made. A significant problem is that improved selectivities for one gas over another are generally obtained at the expense of permeability. Permeability is a measure of the rate at which a particular gas moves through a membrane of standard thickness under a standard pressure difference. Permeability depends both on the solubility of the permeating gas in the polymer and its diffusion coefficient. In general, better selectivities are obtained with glassy or crystalline polymers, because the diffusion coefficient in this type of material is more dependent on molecular size than it is with rubbery materials. Therefore separations on the basis of molecular size are possible. For instance, glassy polymers are typically considerably more selective to hydrogen than to other gases, and in fact hydrogen separation is a commercial application for polysulfone, cellulose acetate and polyimide membranes. However, because they are rigid and inflexible, glassy polymer membranes are typified by low fluxes. On the other hand, rubbery or elastomeric materials with flexible polymer chains are relatively permeable to many gases, but not very selective for one gas over another. To date silicone rubber and other elastomers have found application more as high-flux sealing layers, as in the Monsanto Prism membrane, than as permselective materials. In all composite membranes, whether they include glassy or rubbery coatings, or both, the flux through the permselective layer increases as the thickness of that layer decreases. It is therefore desirable to make the permselective layer very thin. This also present problems, in that very thin, defect-free coatings are difficult to make.
Membrane materials that can give better separation performance, i.e. high flux and high selectivity, are needed. To be useful as the permselective component of a composite membrane, a polymer must not only have good intrinsic gas permeability and selectivity, but must also have the appropriate physical or chemical properties to enable the composite to be made. That is, it must be capable of deposition on the support membrane by a process amenable to large-scale production, without damage to the support membrane, and should be able to withstand the normal operating conditions of that membrane. New polymeric materials with good intrinsic permeability and selectivity properties must be sought. Even when such a polymer is found, however, it may not possess the other necessary characteristics for a composite membrane material, or the technology required to utilize it may be lacking.